The gearbox is the same, Owen, but the clutch has an extra plate so the chain wheel is a bit wider and therefore the clutch push rod has to be longer. That's why the 350 and 250 push rods have a different part number.

There is so little adjustment on the lock nut and screw adjusters in these early three-speed boxes that the push rod length is quite critical to get the lever and cable angles right for the best clutch action.